1. Field of the Invention
The present invention relates to a toner for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like, a developer comprising the toner, a container containing the toner, a process cartridge using the toner, an image forming apparatus using the toner, and an image forming process using the toner.
2. Description of the Related Art
Carbon black, being utilized conventionally as a black pigment in electrophotography, is concerned about its carcinogenicity; therefore, development of a black pigment is demanded that does not cause a safety issue. As for black pigments other than carbon black, there exist magnetic materials for a magnetic toner; however, such magnetic materials are difficult to be utilized for a conventional non-magnetic toner due to its magnetic property, therefore, a black pigment of non-magnetic type is desired.
As for such a black pigment of non-magnetic type, there exist organic and inorganic pigments. Examples of the organic pigment include active carbon, Nigrosine pigment, aniline black pigment and the like. However, such pigments other than carbon black are not sufficiently available, since they exhibit poor coloring power and also decrease toner resistance due to their own low resistance. On the other hand, as for the inorganic black pigment, a polycrystalline particle pigment, having a mixed composition of Fe2TiO5 and Fe2O3—FeTiO3 solid solution is proposed for example (see Japanese Patent No. 2,736,680). This proposed pigment does not cause a safety issue, and is proper in thermal resistance and processing ability. However, the proposed inorganic black pigment is inferior to carbon black in coloring power, therefore, the content of the pigment must be increased up to 40 to 60 wt % in order to maintain the coloring level to that of carbon black.
Further, in the case that a metal oxide pigment is employed as an inorganic black pigment, the specific gravity (absolute density) of pigment itself exhibits two to three times as large as carbon black; the toner containing the inorganic pigment at the content of 40 to 60% by mass exhibits the specific density (defined later) of 1.4 to 1.8 times as large as the toner based on the non-magnetic toner. Accordingly, in a non-magnetic process, such problems arise as premature carrier life due to rapid toner adhesion on carrier surfaces, abnormality on toner content sensors, erroneous detection on remaining amount in one-component toner, and premature life of developing rollers due to toner filming on the developing rollers.
Further, in the case that a metal oxide pigment is employed, a problem arises that the toner tends to be consumed at higher rate, since the adhered amount of toner is relatively high on the OPC (Organic Photo-conductor).
Further, a process for enhancing the coloring power and black level of non-magnetic pigment is proposed, in which a complex oxide of fine particles consisting of cobalt, manganese and iron oxides is employed, the specific surface area of the pigment is increased up to 50 to 100 m2/g, thereby the primary particle size of the pigment is remarkably decreased (see Japanese Patent No. 2,997,206). In this process, the coloring power of the pigment itself may be enhanced, thereby the coloring power equivalent to carbon black may be attained at the content of 10 to 30% by mass. However, such a pigment is not sufficiently available, since the coagulation of the pigment is significant due to the decreased particle size, resulting in the deterioration of charging level due to the insufficient dispersion of the pigment.
Further, a process for improving pulverizability is proposed, in which a foaming agent is added to the toner raw material, then the foaming agent is expanded at the step of melting and kneading to produce a toner (see Japanese Patent Application Laid-Open No. 2000-19775). In this proposal, pores are included in the non-magnetic toner based on carbon black in 10 to 60% by volume. Consequently, the specific density of the toner comes to about 0.7 g/cm3, when 5 parts by mass of carbon black is added to the raw material and when the pore content is 30% by volume; that is, the specific density of toner is excessively low. This results in a lower controllability of toner control sensors, and causes a problem that the controlling part of the apparatus in use must be adjusted with the supplement of the toner.
The object of the present invention is to provide a toner, in which the toner contains a black metal oxide as a colorant, and exhibits a specific density that is equivalent to the conventional toner that contains carbon black, thereby the toner does not impose the necessity to adjust the controllability of toner control sensors, and the supplement of the toner may be easily carried out on the apparatus in use. Also the object of the present invention is to provide a developer, a container, a process cartridge, an image forming apparatus, and an image forming process that utilize the toner respectively.